Authentication Using Client Certificates

SSLVerifyClient require SSLCACertificateFile \     /usr/local/ssl/openssl/certs/ca.crt

Once you have installed certificates in your clients, you need to instruct Apache to enable SSL validation for clients. This SSLVerifyClient directive requires clients to provide a valid client certificate to be able to connect. The SSLCACertificateFile provides the path to the file containing the trusted CA certificate that will be used to verify the client certificate validity.